Known in the art is an internal combustion engine provided with a first exhaust passage and second exhaust passage branched from a common exhaust passage, providing an NOx absorbent storing NOx in the exhaust gas when the air-fuel ratio of inflowing exhaust gas is a lean air-fuel ratio and releasing stored NOx when the air-fuel ratio of inflowing exhaust gas is a rich air-fuel ratio in each of the first exhaust passage and second exhaust passage, providing a fuel addition valve in each of the first exhaust passage and second exhaust passage upstream of the NOx absorbent, and providing an exhaust control valve in each of the first exhaust passage and second exhaust passage downstream of the NOx absorbent (see for example Japanese Patent Publication (A) No. 2003-74328).
In this internal combustion engine, when the stored NOx is released from the NOx absorbent provided in the first exhaust passage, the exhaust control valve provided in the first exhaust passage is closed and fuel is added from the fuel addition valve provided in the first exhaust passage in the state with the exhaust gas standing in the first exhaust passage so as to maintain the air-fuel ratio of the exhaust gas in the first exhaust passage rich, and when the stored NOx is released from the NOx absorbent provided in the second exhaust passage, the exhaust control valve provided in the second exhaust passage is closed and fuel is added from the fuel addition valve provided in the second exhaust passage in the state with the exhaust gas standing in the second exhaust passage, whereby the air-fuel ratio of the exhaust gas in the second exhaust passage is kept rich.
However, in this internal combustion engine, there is the problem that two fuel addition valves are required for making the NOx absorbents release NOx. Further, a big problem is that these fuel addition valves are arranged considerably far from the engine, so it is necessary to lay fuel feed pipes to a considerable distance from the engine.